Demand for high-speed data transmission is ever increasing. Internet access, electronic commerce, Internet protocol telephony, and videoconferencing are telecommunications examples driving such a demand.
Digital subscriber line (DSL) technology provides high-speed data transmission over a so-called “last mile” of “local loop” of a telephone network via copper twisted wire pair cable between residential and small business sites and telephone company central offices and remote terminals. There are various types of DSL such as asymmetric DSL, high bit-rate DSL, single-line DSL, very-high-data-rate DSL, integrated services digital network DSL, and rate-adaptive DSL having various transmission rates, switched circuit characteristics, and other known operation characteristics. These are collectively referred to as XDSL technologies.
In a simplified general view, a DSL system may be considered as a pair of communicating modems, one of which is located at a home or office computer, and the other of which is located at a network control site, typically at a telephone company central office or a remote terminal. The central office or remote terminal modem is connected to some type of network, usually referred to as a backbone network, which is in communication with other communication paths by way of routers or digital subscriber line access multiplexers (DSLAMs). Through DSLAMs the backbone network is able to communicate with dedicated information sources and with the Internet. As a result, information accessible to the backbone network may be communicated between the central office or remote terminal modem and a customer site modem.
DSL applications may be served from central office and remote terminal locations by up to 12,000 feet of copper twisted wire pair cable that may exist between the DSLAM equipment at a central office or remote terminal and a DSL modem at a customer site. However, cable from a remote terminal is typically exposed to a more hostile electrical environment that can cause service reliability problems. These problems become highly prevalent in areas of high earth resistivity, which is usually in soil equal to or greater than 500 meter ohms.
Additionally, although most telecommunication networks have a primary-line protector that is allocated for each customer site as well as for the central offices and remote terminals, the primary-line protector does not protect against induced voltages that are less than 300 volts. Steady-state induced voltages of 20-30 volts can cause signaling and equipment malfunctions to the DSLAM equipment and to the DSL modem as well as reducing advertised transmission line speeds. Worse yet, surge-induced voltages and resulting induced currents exist on the above mentioned wire-line style cable applications when carrying a communication signal between various locations, and may also exist under the operation of the primary-line protector. The surge-induced voltages can cause damage to sensitive electronic components in the DSLAM equipment, such as a line card and the DSL modem equipment, rendering the service inoperable. Also, impulse noise spikes can occur that can reduce the effective speed of the data transmission.
Historically, AC induction problems have come from the long loops serving customers at the ends of an exchange area boundary, which are the most distant customer terminal locations within local calling areas from a central office. These loops are not only predominantly exposed to unbalanced, single phase power lines, but the areas they serve are in suburban/rural environments that are less “built-up”, and additional shielding benefits are not available as in urban areas.
Neutralizing transformers were originally designed for use in open wire telephony networks. Large oil-filled neutralizing transformers have been used on wire-line facilities entering power substations and in generating plants to suppress high-induced alternating current (AC) voltages and ground potential rise (GPR). Smaller dry-type neutralizing transformers, known as induction neutralizing transformers (INTs), have been used to reduce voice-frequency noise and induced AC voltages from 350-600 volts on wire-line voice-grade and digital carrier telecommunications circuits.
However, the voice-grade INTs are typically made with up to 500 feet of 26 gauge Category 3 cable, which not only reduces the available transmission overhead margins which limits their application on DSL applications, but also increases the probability of crosstalk coupling interference on adjacent DSL circuits. As the length of a copper pair is extended, the signal power decreases in intensity, thereby, limiting the allowable distance between a customer terminal and a central office or remote terminal. Also, the higher the frequency application, as with DSL, the more noticeable the diminution in signal power. The use of a voice-grade INT reduces signal power approximately 2.3 dB at lower DSL frequencies.
Digital-grade INTs are typically wound with up to 200 feet of 24 gauge Category 3 cable, and are built with a screen (shield) to separate two directions of T1 carrier transmissions, to prevent crosstalk interference. Digital grade INTs exhibit a lower signal power loss over voice-grade INTs, but as with voice-grade INTs they are large, heavy, and expensive units because of the 350-600 volt design criteria. The costs involved in manufacturing and implementing both digital-grade INTs and voice-grade INTs is high. The screen is a significant added cost and is not required in DSL applications.
An additional impairment with high-speed DSL transmission speeds is crosstalk interference between adjacent circuits. The crosstalk can be minimized by using non-adjacent pairs within a binder group. However, the non-adjacent pairs become tightly wound together for several hundred feet in an INT. Coil-winding of the INT tends to spread the Category 3 cable wire pairs out and thus increases the probability of crosstalk interference between the closely coupled pairs.
It would therefore be desirable to develop a high-speed digital telecommunication network containing voltage neutralizing devices that reduce lower level steady-state and surge induced voltages and that are suitable for DSL applications, including being smaller in size, lighter in weight, and less expensive to manufacture and implement as compared to traditional INTs. In so doing, a more reliable telecommunication network is created to satisfy the ever-increasing demand for high-speed communication at customer terminals that are at great distances from a central office.